Current commercial processes for the preparation of 2-pyrrolidone (2-pyrol) can be carried out in the liquid phase under a pressure of about 2,000 psig as is disclosed in U.S. Pat. No. 3,350,784 and British Patent 1,312,463; however liquid phase processes are generally associated with low pyrrolidone (pyrol) product yield, about 10% of the starting material being lost to dehydrated pyrol dimer by-product and other by-products which are difficult to analyze. Most often those impurities are included in the yield of 2-pyrol reported by the art. Liquid phase processes are also associated with the formation of other contaminating by-products such as 4-(N-pyrrolidonyl) butyramide (PBA), tetrahydrofuran and hydroxyalkyl butyramides which are derived from 4-hydroxy butyramide (HBA) intermediate. The formation of significant amounts of PBA impurity in current processes unavoidably takes place because of the cleavage of the lactone ring to form the intermediate which subsequently undergoes ring closure with the elimination of water to produce a crude product. It has been found that high dissolution of the reaction mixture may reduce the formation of by-product to some extent; however, this solution is not commercially attractive since a significantly larger reactor capacity is required and added costs are incurred by diluent separation.
Other objections to the liquid phase reaction involve lengthly reaction time of from 6 to 10 hours. To overcome some of the above difficulties, vapor phase reactions of lactone and ammonia have been proposed, examples of which are illustrated in U.S. Pat. Nos. 4,824,967 and 3,136,780. However, these vapor phase reactions require the use of a particulate silicate catalyst which requires separation and regeneration and in which product, along with impurities, can be entrapped, thus representing a considerable loss in recoverable product.
Although some prior processes achieve high conversion of butyrolactone, the crude product has been found to be highly contaminated with the above impurities and may also contain other contaminants which cannot be removed by fractional distillation and which require an additional acid treatment for product purification, as described in U.S. Pat. 3,140,294.
Accordingly, it is an object of this invention to provide a commercially feasible liquid phase process for the reaction of butyrolactone and ammonia to produce 2-pyrrolidone in high yield and selectivity within a relatively short reaction period.
Another object of the invention is to provide a process for the preparation of 2-pyrrolidone which minimizes contamination so that, for certain purposes, the separation of unwanted by-products may not be necessary.
Still another object is to provide a process which may be carried out in a batch or a continuous method and in the absence of extraneous solvents and catalyst.